Trouble with high kicks

I'm a southpaw, and recently I've started trying to utilize my left high kicks more. Unfortunately, it seems like every time I spar a taller opponent and try them they end up being awkward and without power or I end up off-balance. I don't think it's an issue of flexibility, since I've got a 170-175 degree range of motion, and I never have trouble kicking at head height or higher with power on the heavybag, so I don't think it's an issue of poor technique. Maybe high kicks just don't work on opponents that're taller than you? Or am I missing something here?

If you have that range of motion in your lags and you lack power in your kicks, it is a question of technique!
Ask your coach to watch you when you spar, I think he will find the problem. In most cases people do not turn their hips when they kick, so they block themselves and lose power and range of motion. Also remember you are only going to kick as high as your knee points. So if your knee is pointing towards his hips you are only going to kick with full force up to the hips, everything else will be a trade of, power+speed for more hight.
Also make sure that when you kick your hip is driving "through" the target and does not stop when you are about to hit it. This is another common mistake.
For everything else, see my first advice, ask your trainer because he knows you, he can show you the technique and most important, he can see your kicks, we don't.

I'm gonna second the rotating the hips bit. Sounds you are trying to pick up your knee, balance on your foot and point your kick at his head like a ballerina. No insult intended.

You need to swing for the fences, boyo. Regardless of the human not being stationary like a bag argument, if you can hit it that high on a heavy bag there is no reason why you wouldn't be able to do it to a person, so long as you practice proper distancing.

Anyways, personally- if I'm sparring a taller guy, I don't generally go for head kicks unless he bends his head down alot and I see an opportunity for him to step right into it. If you can kick as high as his head, but don't hit him it's because he has the reach to keep some distance between the two of you, and you are then correct in assuming this puts you off balance and makes you susceptible to sweeps. My strategy against a taller opponent is to chop at them with leg kicks so they fall like a tall tree, than once they get weak, some sharp boxing and clinchwork to seal the deal and end it.

Did your instructor specifically say to work on high kicks during this sparring session?

"This is why we are here. Because the Martial Arts for too long have been cloaked in an unnecessary level of secrecy bordering on mysticism, and its in these shadows that the cockroaches love to hide. -Phrost"

You may also want to consider WHEN you are throwing your high kicks and what you are doing immediately before . You should not be throwing high kicks right off the bat , you need to mix them up with your hands .

and as Omega nicely stated :

VIDEO BITCH !

and kindly asked

WHAT FUCKING KICK ARE YOU USING ?

Originally Posted by Bodhi108

Nuke a unborn gay whale for Christ.

“I don't mean to sound bitter, cold, or cruel, but I am, so that's how it comes out.”
BILL HICKS,
1961-1994

You need to swing for the fences, boyo. Regardless of the human not being stationary like a bag argument, if you can hit it that high on a heavy bag there is no reason why you wouldn't be able to do it to a person, so long as you practice proper distancing.

Miss enough times and you can lose confidence in the kick which will make you hesitant.